There are many different types of vessels that can be classified as submarines or submersibles. A submarine is typically considered an autonomous vessel, capable of moving forward and changing directions under water, capable of navigation on the high seas, with seakeeping capabilities, and capable of safely operating under water. A submersible is generally considered any vessel that can submerge and operate underwater, but may have limited or no capacity to navigate the seas on its own. Submarines and submersibles both carry human passengers under the surface of the water. Therefore, any submarine or submersible must at least be able to attain negative or neutral buoyancy, and provide propulsion for the passengers. Most submarines and submersibles also provide some sort of life support for the passengers, though some submersibles may require the passengers to wear Self-Contained Underwater Breathing Apparatus (SCUBA) gear. All submarines, and many submersibles, keep the passengers safe from the pressure of the water at depth.
Buoyancy is the upward force exerted by water on a submerged or partially submerged object. A vessel will float in water when the buoyant force pushing up is equal to the gravitational force pulling the vessel down. The buoyant force pushing up on an object in water is called hydrostatic lift. The magnitude of the buoyant force, or hydrostatic lift, is dependent of the amount of water displaced by a particular object. When an object is on the surface of water, gravity pulls it down, and if the bottom is sealed, as in a vessel, it will push water aside. The volume of water displaced will be equal to the volume of the object that is below the waterline, which is known as the surface displacement. The buoyant force acting on an object is equal to the weight of the volume of water displaced.
Since the displacement of a vessel on the surface of the water is dependent on its weight, it can be controlled by using ballast. Ballast is usually water that is allowed to enter a vessel into sealed hull compartments.
In surface ships, water is typically added to ballast compartments to add additional mass to the lower portions of the ship. This lowers the ship's center of gravity and therefore increases its stability on the surface. Submarines and submersibles have historically not needed ballast water for stability on the surface since they will usually already have a low center of gravity and little cargo variability. They also typically sit very low in the water while surfaced, with only a very small volume of the overall vessel above the waterline.
In submarines and submersibles, water is added to ballast compartments to help them sink below the surface. This can be looked at as either reducing the displacement of a vessel or increasing its weight; both have the same mathematical effect. In submarine terminology, adding ballast water is typically viewed as reducing the vessel's buoyancy. When ballast compartments are full, they are looked at as being essentially neutrally buoyant, and thus accounting for no buoyant force on the vessel. The mass of these ballast compartments must still be considered in the energy required to propel the vessel underwater. These ballast compartments are often called variable displacement, since they allow water to enter and reduce the buoyancy of a vessel by reducing its displacement.
For vessels capable of underwater operation, such as submarines and submersibles, they must attain neutral buoyancy by adjusting the amount of ballast water. Neutral buoyancy refers to the condition where the upward force of buoyancy equals the downward pull of gravity. In this condition, a vessel can use its propulsion systems to rise, sink, or move about in the water.
In typical submarines, the weight of the vessel is set such that it is just enough to overcome the buoyant force due to the vessel's fixed displacement. The fixed displacement is the volume of the portions of the sub that are watertight and cannot be flooded, which determines the minimum buoyancy. This setting of the weight is necessary to allow the vessel to submerge and attain neutral buoyancy even when it has no extra weight by completely flooding the controllable ballast. Thus, the variable displacement is determinant of its payload capacity. The fixed displacement in most submarines is contained mostly within the pressure hull, the strengthened passenger compartment that resists the extreme pressure of the water at depth.
Typical small submersibles are used for either deep-diving science or industry missions or for shallow-diving recreational trips. Both types have the minimal controllable ballast necessary to reach the buoyancy needed to barely rise above the surface just enough to allow passengers to enter and exit the vessel. The variable displacement is extremely small compared to the amount of fixed displacement in these vessels. Due to this fact, these small submersibles are not able to rise high enough above the surface to allow a majority of their volume to reside above the waterline. They are not able to achieve the amount of positive buoyancy necessary to do so. Keeping the majority of the volume of the vessel underwater when surfaced simplifies the design of the vessel and keeps it stable both underwater and while surfaced. However, this design approach gives very limited ability to travel on the surface and allows a very low degree of variability in payload.
While originally developed for military use, vessels capable of underwater operation are used today for a wide variety of purposes. Modern submarines and submersibles are very specialized, though, varying from vessels that serve as military weapons platforms to those that are used for deep-sea scientific research to those that are used for recreational shallow dives. Despite a large quantity of submarines and submersibles for many specialized tasks, no true general-purpose vessel capable of underwater operation exists that can be used for multiple applications. There is also currently no underwater vessel that is small, cost-efficient, and possesses significant autonomy, navigation capacity, and range. The only submarines in use today that can navigate on the high seas are huge, ship-sized military submarines. A small vessel capable of underwater operation with true navigation capacity and sea-keeping abilities would be extremely useful in both private industry and to the military.
The designs and capabilities of existing submarines and submersibles are varied, from simple underwater scooters for divers to the huge nuclear missile-armed boomers of the United States Navy. Nevertheless, all existing submarines and submersibles share the common trait that they are each designed for a very specific purpose and have limited utility in applications outside of their intended use.
Nearly all of the world's true submarines today are large, ship-sized military vessels. In fact, the only true submarines currently in use are the transoceanic military vessels used by the U.S. Navy and other industrialized nations. These are among the most sophisticated vessels capable of underwater operation ever built.
Some nations still use diesel-electric military submarines, which were in wide use during World War II. The modern versions display better submerged time, speed, stealth, and armament, but are similar in basic function. They use surface engines to charge electric batteries for a dive. Nuclear-powered designs that can spend months underwater have replaced many diesel-electric submarines in the U.S. Navy and in some foreign navies. The atomic reactors used to power these vessels can operate for years without needing to add fuel, and they typically only need to surface every few months to add supplies and exchange crews.
Transoceanic military submarines are used for many different missions, and each mission type is usually accomplished by a particular class of submarine. The U.S. Navy currently has the Ohio-Class Submersible Ship Ballistic missile Nuclear (SSBN), the Submersible Ship Guided missile Nuclear (SSGN)-Class, the Los Angeles and Seawolf Submersible Ship Nuclear (SSN) classes, and the Virginia SSN class. The Ohio-Class SSBN, also known as boomers, serve as a stealthy mobile launch platform for ballistic missiles. The SSGN-Class are boomers that are converted to carry cruise missiles and to serve as platforms for Special Forces operations. The L.A. and Seawolf SSN classes are fast attack submarines, and the Virginia SSN class is a fast attack sub, cruise missile launch platform, and Special Forces platform. All U.S. Navy submarines are atomic-powered and capable of diving to at least 800 feet below the surface of the water.
Transoceanic military submarines are designed for long-range cruising at relatively high speeds compared to surface vessels and for stealth. These vessels are typically as big as large surface ships. None of these military submarines have any non-military use. They are far too expensive and impractical to transport cargo. Passenger travel is cheaper, faster, more practical, and more comfortable by other means. Industrial use is not practical for these submarines since their size prevents operation around or under other vessels or offshore platforms. The lack of windows, large crew size needed for operation, and huge cost to produce precludes any tourism or recreational use. Thus, these huge military submarines have no other use besides their current warfare platform.
There are many existing submersibles collectively capable of a variety of different uses, but each individually only useful for a specific task. Smaller submersibles are used by both the military and private sector and are generally characterized by the fact that they are not used for navigation on the high seas but rather are vehicles used to access the depths of the ocean in a largely vertical range.
The most basic submersibles are pods that are lowered and raised in the water by a surface vessel using a support cable. Diving bells and bathyspheres are examples of these simple pods.
A diving bell is basically a submerged pocket of air in an air-tight compartment that is partially open at the bottom. The bottom may either have a hole in the center or a hatch that is opened when the diving bell is submerged at depth. A diving bell acts as an elevator taking divers deep under the surface of the water and as a decompression chamber slowly bringing divers back to the surface. A diving bell resists the extreme pressures at depth through the use of ambient pressure compensation. As the diving bell descends under water, the water pressure outside the diving bell increases. If no air is added to the diving bell, the water ingresses through the opening and begins filling the bell. The air trapped in the diving bell compresses at the top until it reaches the ambient pressure of the water outside. When the pressure on the inside and outside of the diving bell is equal, water stops intruding into the diving bell. As the diving bell continues to descend, the pressure becomes greater, the air compresses more, and the air pocket becomes smaller. Since the pressure is equal on the interior and exterior of the diving bell, the water does not cause any stress on the wall as long as the diving bell remains open. This means that common materials of low strength may be used to construct a diving bell, as long as they are reasonably airtight.
Typical modern diving bells are designed to remain dry on the inside. Compressed air is released into the diving bell at depth to prevent water ingress at depth. The air is usually provided by a surface support vessel via an umbilical connection. Air is provided at a pressure just higher than that of the water outside the diving bell. This causes air to slowly bubble out through the diving bell opening and keeps the air supply fresh for the passengers. When the diving bell ascends to the surface, the pressure of the water decreases and the air inside expands and bubbles out.
The main limitation of any ambient pressure submersible, including a diving bell, is a result of the limitations of the human passengers. The body is stressed by increases in ambient pressure but compensates by pulling more air into the lungs, increasing the amount of gases in the blood. If too much nitrogen enters the blood, narcosis may result. This threat can be somewhat eased by mixing other inert gases in the breathing mix, such as helium. Breathing mixes usually also have a lower percentage of oxygen than normal surface air since the compression causes additional oxygen to enter the blood. As the pressure drops when the ambient pressure submersible rises, the body must expel the excess gases accumulated during the compression dive though exhalation. This is a slow process; the longer time spent at depth, and the deeper the depth, the longer the expulsion of excess gases takes. If the ambient pressure submersible rises too quickly, the gases in the blood can bubble and cause “the bends,” which is very painful and can result in a fatal embolism.
Therefore, passengers of an ambient pressure submersible must decompress as they surface, just as a SCUBA diver must. This limits the usefulness of ambient pressure submersibles to the same depths that SCUBA divers can reach. This a maximum of about 200 feet, with about 33 feet being a more practical for passengers that are not expert divers.
To allow for long duration dives and rapid ascent, and to protect passengers from dangerous high pressure conditions common to ambient pressure submersibles, the passenger compartment must be kept at the normal air pressure, one atmosphere. The pressure from the water tends to crush the passenger compartment more and more as the depth increases, though. The passenger compartment must be constructed in a strong, pressure-resistant manner.
A pressure hull is a manned pod constructed of extremely strong and durable materials capable of resisting the crushing force of water at depth and protecting passengers without ambient pressure compensation. Pressure hulls are usually spherical or cylindrical in shape since these shapes tend to be inherently resistant to compressive force. Any deviation from these shapes greatly reduces the pressure tolerance and thus the maximum depth reachable by the hull. Pressure hulls are therefore constructed with a high degree of precision in shape. These precise tolerances increase the time and expense of constructing the hull.
A bathysphere is a simple pressure hull suspended from a cable. It usually has a viewing window and accommodations for a crew inside. Stored oxygen and a carbon dioxide scrubber are commonly used for life support. Bathyspheres were the first submersibles to carry humans to depths over 3,000 feet below the surface of the water, and were originally used for scientific research. They are not used very much any more.
Diving bells and bathyspheres are both limited by the heavy steel cable connecting them to the surface or a surface vessel and by their complete lack of autonomy, both on and below the surface. A large surface vessel, with its own crew and cost is needed to provide the support cable or umbilical. The lack of self-propulsion, power storage, and buoyancy control, along with the hindrance of the large cable, prevent either of these from being submersible vehicles. Instead, they are useful for operations or observation only. The bathysphere is limited to a depth of about 3,500 feet while diving bells become dangerous deeper than around 300 feet.
Deep Submergence Vehicles (DSVs) are designed to reach the deepest portions of the ocean. A relatively small number are in existence and have been used for scientific and military research purposes. DSVs usually require a support vessel and cannot navigate. DSVs have two categories: bathyscaphes and deep-dive submersibles.
The bathyscaphe is an old vessel no longer in production, and very few have ever been built, likely less than 10. A bathyscaphe was the vessel used to reach the deepest point on Earth, the Challenger Deep portion of the Marianas Trench in the South Pacific Ocean. A bathyscaphe is a spherical pressure hull suspended from a buoyant superstructure filled with petroleum fuel. The fuel is not used for power, but rather to provide resistance to compression at depth. The fuel also provides buoyancy control. To descend, fuel is released to reduce buoyancy. To ascend, metal pellets are released from the vessel to reduce weight. The pellets are held in place in hoppers via electromagnets, meaning that electrical failure would result in the vessel immediately rising towards the surface. Battery-powered electric thrusters provide propulsion and steering under water, but this capability is extremely limited due to the large superstructure.
While being able to dive to great depths, the bathyscaphe is very limited by its size and low maneuverability under water. It is also difficult to launch and recover. No bathyscaphes are currently known to be in operation.
Deep-dive submersibles are small battery-powered submersibles with a spherical steel pressure hull. They are similar to bathyscaphes, but are smaller and without the fuel-filled superstructure. The pressure hull is typically thinner than that of a bathyscaphe, resulting in a lower maximum depth. The lower weight allows ascension to the surface to be achieved without the fuel superstructure of the bathyscaphe. Deep-dive submersibles rise using the buoyancy of the pressure hull along with high-pressure air-blown buoyancy tanks and oil-filled equipment chambers or high-strength glass-bead foam blocks.
Deep-dive submersibles have small ballast tanks that fill with air at the surface to allow the vessel to have a small portion of its volume above the waterline. The ballast tanks flood to cause the vessel to dive, and the tanks remain open at the bottom, maintaining ambient pressure at depth. These vessels are typically negatively buoyant when diving, using the force of gravity to sink until the desired depth is reached. Then, the vessel can drop weight and add high-pressure air to the ballast tanks to achieve neutral buoyancy. To rise back to the surface, deep-dive submersibles drop disposable metal ballast and do not typically need electric propulsion. Battery-powered electric motors are used for limited underwater propulsion and steering. Deep-dive submersibles are more maneuverable than bathyscaphes due to their smaller size.
The reliance on drop weight to return to the surface can be a problem for deep-dive submersibles. If reconfiguration is desired or if a weight needs to be lifted during a dive, this can be difficult to achieve. Deep-dive submersibles have a low weight budget for ascending, since there is little variability in their buoyancy and minimal drop weight. The air-blown ballast tanks contain only a small fraction of the displacement of the passenger compartment and provide very minimal adjustable buoyancy.
Extreme safety precautions and precise engineering are necessary with a deep-dive submersible. Since the pressure hull's displacement is needed to rise to the surface, any flooding of the pressure hull will cause the vessel to sink to the bottom. The precise engineering necessary to help avoid this risk increases the cost of production.
DSVs have little or no navigating ability or range, since they use battery-powered thrusters and have the majority of their volume under water when surfaced. A surface vessel is needed to deliver them to and retrieve them from a dive site. Huge cranes are often needed to lower DSVs into the water.
Additionally, DSVs have very small passenger compartments. The spherical pressure hulls are usually designed with the minimum radius necessary to accommodate a crew and vital instruments. The U.S. Navy has one DSV that is larger than most typical DSVs, but it is too expensive to be practical for non-military purposes. Overall, DSVs are useful for a very small range of tasks but are severely limited by their lack of range, seakeeping ability, autonomy, and speed.
Another existing common type of submersible is the tourist submersible. Tourist submersibles are some of the largest private submersibles, often accommodating 16 or more passengers. They usually have a pressure hull and operate at a depth of between 1-300 feet of water. The pressure hull is usually large, elongated, and made of steel with some oversized hemispherical acrylic viewing windows. Tourist submersibles are powered by large battery arrays located in the keel and are propelled by electric thrusters.
Tourist submersibles are not useful for purposes other than sightseeing. They lack speed, autonomy, and navigation capability. They only have a small portion of their volume above the waterline when surfaced. They are dependent on battery power and air stores, which require a support vessel or dock to recharge. Tourist submersibles must be large in size to be cost effective, but their size causes them to be depth-limited due to the large force exerted on the pressure hull.
Tourist submersibles use the buoyancy of the passenger compartment to rise to the surface, similar to DSVs. Thus, any failure of a pressure hull penetration could cause the submersible to sink to the bottom. This leads to increased engineering costs.
Some tourist submersibles have been manufactured with small diesel engines giving them the autonomy to go to a dive site and back without a support vessel. However, these vessels still lack open-ocean navigation ability and are very limited in their range. They are also unsafe to operate in even moderately rough seas and still suffer from the problems of other tourist submersibles with respect to depth, speed, size, and cost.
Ambient pressure personal submersibles resist pressure at depth using ambient pressure compensation as a diving bell does. This limits their use to depths that a SCUBA diver can reach. The simplest ambient pressure personal submersible is a wet hull submersible.
A wet hull submersible is an underwater vessel where the passengers are exposed to the water while the vessel propels them through the water. They typically have small ballast tanks to help attain neutral buoyancy and electric motors for propulsion. The passengers are supplied air via an air-filled helmet or a breathing apparatus such as SCUBA gear.
Wet hull submersibles are obviously very limited in their use. The passengers are exposed to the water temperature, which can be problematic in cold climates. The passengers are also exposed to pressure at depth, which means wet hull submersibles are limited to depths of around 200 feet even when manned by expert divers using mixed gas for breathing. A depth limit of 33 feet is more practical for sport divers.
The military uses a wet hull submersible called a Seal Delivery Vehicle (SDV). The passenger compartment is completely enclosed, but still flooded. Thus, the SDV suffers from the same shortcomings as other wet hull submersibles.
An ambient pressure dry hull is a submersible with the hull sealed so that the interior is dry. A gauge is used to determine the ambient pressure of the water outside. Air is added to the passenger compartment through valves until the interior pressure equals the exterior pressure of the water. A check valve is used to release the air as the submersible rises to the surface and the water pressure decreases. The hull can be constricted in any reasonable shape and any material that is reasonably airtight.
Ambient pressure dry hulls are depth limited by many factors. First of all, the amount of air and battery power reserves they carry prevents them from going too deep. More importantly, though, they are limited by the human body. A depth of about 200 feet can be achieved with highly trained divers, but a more practical depth limit is 33 feet below the surface of the water.
Ambient pressure personal submersibles are generally battery-powered. They have little surface range, seafaring capability, or autonomy. They rely on surface vessels to reach a dive site and to recharge their batteries and air supply. A few ambient pressure personal submersibles have been built with diesel surface engines, but they still face the same depth limitations as all other ambient pressure designs.
The Advanced Seal Delivery System (ASDS) is a submersible that uses a pressure hull for Navy Seal operations. It is relatively large for a submersible, at 62 feet long. The ASDS is a highly special-purpose vessel, designed for stealth utility. It uses only battery power for propulsion, which severely limits its range and seafaring abilities. The ASDS has low amounts of buoyancy for rising to the surface, and instead commonly docks underwater with a host submarine. The cost of these submersibles is extremely high, meaning they have no non-military use.
Every existing submarine and submersible is designed, built, and used for a specific role. Thus, there exists a need for general purpose submarine. Such a submarine would be capable of performing many roles under water and able to navigate on the surface, with strong seakeeping ability, long range, high speed, and autonomy. Such a vessel would be useful for both the private sector and the military. There are many characteristics and capabilities a general purpose submarine should possess.
A general purpose submarine will need to be relatively small in size but should still be able to accommodate passengers. Submarines have historically been impractical for long-range cargo or passenger transport. Sightseeing, industrial, and security uses are all good uses for a submarine, but all require small submarines relative to the atomic-powered military behemoths. Small size is also key to reducing the cost of producing, operating, and maintaining a submarine. Smaller pressure hulls allow submarines to operate at greater depths than their larger counterparts since the water pressure is spread over a smaller surface area on the hull. Good general purpose submarines should be able to accommodate a crew or passenger contingent of from as few as one to as many as 12 people during a dive. In most cases, this should be accomplished by use of various sizes of a one-atmosphere pressure hull providing safety and rapid ascension. This capacity is useful for scientific, military, recreational, industrial, and other uses.
A general purpose submarine should be a capable navigator with a long operating range and strong seakeeping abilities. It should also be autonomous, capable of generating its own power and air supply and storing them for a dive. Such a vessel would be more efficient and less expensive than a vessel that requires a surface support vessel. The safety factor in rough seas would also be higher since it could ride out the storm without the assistance of a surface support vessel. The ability to navigate over a long range would allow private industry to be able dispense with the cost of a surface support vessel for the first time. The submarine could deploy from a regular dock and travel to its destination on its own.
A good general purpose submarine should be able to attain decent speed, as traveling at high speeds is useful in many circumstances. Speed allows a tourist sub to carry more passengers without the need for a surface support vessel. Speed is also useful in military and security operations. Further, speed lowers mission duration, which decreases costs, and allows a vessel to avoid approaching storms.
A general purpose submarine should be configurable, possessing the ability to take on equipment needed for several different types of missions. Configurability is a key feature of a general purpose sub. Such a vessel should be able to increase passenger compartment comfort when used in a recreational or tourist role; have weaponry and armor added to it in a military role; and be equipped with cameras, manipulators, storage, and tools in a scientific or industrial role. These reconfigurations should not require substantial redesigning of the vessel; ideally these reconfigurations should not significantly increase the cost and time needed to deploy these variations of the general purpose submarine.
A general purpose submarine should be capable of diving and keeping passengers safe at depths that encompass the majority of water that is useful to industry and tourism. While for some purposes, such as tourism, depths of 33 feet may be sufficient, in many embodiments, such a vessel should be able to dive to at least 500 feet, covering about 90% of the useful water. These depths are sufficient to offer stealth to the military as it exceeds the depth at which light penetrates the water in most locations, and even long-range military submarines rarely operate below depths of 1,000 feet. For industrial operations, the majority of oil pipelines and infrastructure lie in the first 300 feet of water.
The amount of time a general purpose submarine should be able to sustain a dive should allow the maintenance of depth for at least a full work day. This would allow industrial or scientific operations to accomplish a sufficiently full day's work, and would enable a military user to remain submerged during all daylight hours to remain hidden. Longer dive time capability increases safety as well, as it allows for additional time to rescue a stricken submarine.
Any general purpose submarine should have a high degree of safety. Due to the fact that it will be autonomous and capable of navigating the high seas on its own, safety becomes particularly important. Such a vessel should be capable of handling severe weather conditions and unforeseen circumstances on the surface, as well as multiple system failure during a dive.
A general purpose submarine should be able to be constructed at low cost. The production and operating costs should not exceed the combination of costs of a surface ship and a submersible, or of a surface ship with a Remotely Operated Vehicle (ROV).
Many challenges must be overcome to design and build a general purpose submarine with such characteristics and capabilities as many of those listed above. First of all, small size must be reconciled with high speed, navigation, seakeeping ability and long range.
It has historically proven difficult to design a relatively small submarine capable of long-range navigating and seakeeping at high speed. Typical submarines that do the listed capabilities are extremely large, bulky, and expensive. These submarines have been useful only for the military. Private industry has solved the navigation issue by transporting small submersibles using surface vessels, which is costly and wasteful. However, the desire for long-range navigating and seakeeping at high speed indicate that such a general purpose vessel should be a true submarine, not a simple submersible, thereby making it more difficult to design. The vessel must either be capable of long-range navigation underwater or must be capable of handling as a true surface craft in addition to its ability to operate under water. Long-range underwater navigation has only been successfully accomplished through the use of nuclear power or enormous battery banks charged on the surface by diesel power. These methods are practical in large, transoceanic military submarines, but are not possible in a smaller submarine.
Existing designs of small submersibles are incapable of acting as a true surface craft. Small submersible designs rely on batteries and electrical motors for propulsion. Batteries are very limited in the amount of energy they store, so to operate as a true surface craft, a vessel should carry a large, heavy fuel load. Large, powerful engines should also be present for the submarine to operate capably on the surface. The use of large, powerful engines and large fuel reserves allow high speed, long range, and large payload capacity in surface boats. However, it is not a simple task to add such engines to an existing submersible design. Attempts that have been made to add diesel engines to tourist submersibles or smaller industrial submersibles have been ineffective since their functionality is limited by the design of the submersibles.
The first reason for the failure of the attempts to add large engines and fuel stores to existing designs is due to the increase in weight caused by these additions. Small submersibles typically have small amounts of buoyancy, and extensive modifications are necessary when adding significant weight. Adding a large engine and fuel would lead to the vessel not being able to surface. Such a vessel would obviously have no practical utility. Thus, additional displacement must be added to the vessel to provide additional buoyancy.
Small submersible designs have increased the size of their pressure hulls to provide the added displacement. This protects the engines from pressure and water at depth as well as adding the necessary displacement to allow the vessel to surface. To accommodate the large engine requires a significant increase to the pressure hull, though. Such an increase drastically increases the vessel's weight and fixed displacement (which increases the submersion weight). The net result of such an arrangement is either a decrease or minimal gain in the power-to-weight ratio of the vessel. The massive increases in weight necessary to add power to a small submersible using this method sets up the paradox that the power is self-defeating because of the weight. Therefore, small submersible designers have only been able to include small engines and fuel tanks. The small amounts of power provided severely limit navigation, seakeeping, speed, and range.
The second reason for the failure of the attempts to add large engines and fuel stores to existing submersible designs is because of the hull shape and draft. The typical hull shape of a submersible is cylindrical, optimized for underwater operation and handling at fairly low speeds. When operating on the surface, with the majority of their volume below the waterline, these vessels handle very poorly, incurring significant drag. They also have poor seakeeping ability because of the lack of a sharp bow necessary to pierce waves and handle rough seas. Therefore, increasing the power only minimally increases the speed, since the deep draft and improperly shaped hull result in significant drag and extremely poor handling in rough seas.
Another challenge that must be overcome to design and build a general purpose submarine is reconciling small size with configurability. Existing small submersibles have proven to be very difficult to make configurable. A main reason for this difficulty is the way submersible designs approach buoyancy. Most submersibles are designed with minimal buoyancy when surfaced, leading to very little ability to carry extra weight and still maintain their capability to surface. The passenger compartment must also be very carefully engineered and scrutinized since almost all the submersible's buoyancy comes from it. Any water intrusion into the passenger compartment could cause the submersible to sink and kill the passengers. Adding equipment to the hull typically requires redesigning the entire vessel. Therefore, submersibles are designed with a maximum load, and any reconfiguration requires an extensive redesign. It is too costly, time-consuming, and impractical to reconfigure the vessel for alternative uses once it has been built.
The next challenge to overcome in designing and building a general purpose submarine is reconciling design simplicity and low cost with dive depth and duration. Submersibles that have typically been affordable for private users are ambient-pressure submersibles. These do not require heavy-duty pressure hulls and the engineering challenges that come along with pressure hulls. However, ambient-pressure submersibles are only safe at depths of about 33 feet, and only up to about 200 feet for experienced divers using mixed breathing air.
On the other hand, a pressure hull is needed to allow a vessel to achieve great depth and long duration dives. Existing submersible designs incorporating pressure hulls are subject to catastrophic threats if swamping and leakage occur, and thus they require costly, complex safety engineering. Thus, using existing designs, it is not possible to achieve deep and long duration dives while also keeping the cost low and the design relatively simple.
The next challenge in designing and building a general purpose submarine is reconciling design simplicity and low cost with safety. One of the key problems in designing a simple, low cost submarine has historically been the huge expense that goes into engineering safety into a typical submersible. While ambient-pressure submersibles are relatively low cost and of simple design, they are inherently dangerous and must be operated by trained individuals who understand the process of decompression. Pressure hull designs are inherently safer than ambient-pressure designs since they do not expose the passengers to increases in pressure at depth. Though safer, the pressure hull design requires a lot of high cost engineering to remain safe because of the pressure differential that exists at depth. Additionally, due to the small amount of buoyancy typically present aside from the pressure hull, a failure in the pressure hull will result in the submersible sinking to the bottom in a typical design. Thus, complex engineering and maintenance precautions must be used to ensure safety. This increases the cost. Once again, using current designs, it does not seem possible to design a simple, low cost submarine with a high degree of safety.
Another challenge to overcome for a general purpose submarine to be designed and built is reconciling navigation, high speed, seakeeping, and long range with configurability. A submarine that is able to be configured for multiple roles must be small in size. However, small size is incompatible with traditional notions of what is necessary to achieve long-range navigation and seakeeping abilities at high speed. Additionally, a configurable submarine must be capable of carrying a variable payload.
Speed, range, and navigation and seakeeping abilities require large engines and large fuel stores. The submarine must be capable of carrying this weight. Additionally, configurability increases the payload requirements since the vessel must be able to carry a wide variety of heavy items, such as manipulator arms, weaponry, armor, cabin furnishings, deck space accompaniments, or added instrumentation. The limited amount of displacement, and thus buoyancy, in typical designs prevents this extra weight from being possible since the submersible will not be able to surface if it were to be added.